The purpose of this study is to investigate the effects of number of graphite nodules on fatigue limit and fracture origins in spheroidal graphite cast iron where carbon content and the number of graphite nodules are changed. As specimens, spheroidal graphite cast irons with 3.0, 3.2, 3.4, 3.6 and 3.8 mass% carbon content were produced respectively. Matrix was conducted pearlite (PDI) and ausferrite (ADI) by heat treatment (normalizing and austempering). The mean diameter of the graphite nodule decreases as the carbon content increases, and the number of graphite nodules per unit area increases. No differences were observed in tensile strength as a result of variations in the number of graphite nodules. The rotating bending fatigue test was conformed to JIS (Japanese Industrial Standards). Load frequency was 47 rpm, number of cycle to discontinue test was 1.0 × 10⁷ cycles, and the specimen used was the 1 type of 8.00 mm in diameter. All the fracture surfaces were observed with a scanning electron microscope. The relationship between the characteristics of fatigue strength and the size of fracture origins (defect size) was investigated.
No differences were observed in fatigue limit as a result of variations in the number of graphite nodules. Fracture origins were micro-shrinkage, unspheroidizing graphite nodule and aggregate graphite nodules. The percentage of fracture origin of aggregate graphite nodules was increased by cluster of the graphite nodules. The reason of this phenomenon was due to the decrease of the graphite nodules spacing. The mean defect size in each sample was constant irrespective of the change of number of graphite nodules, which was good agreement with the results of the fatigue limit.